FULL PAPER © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 1603820 (1 of 9) and electrochemical properties of both electron donor and acceptor play impor- tant roles in controlling the morphology, light harvesting properties, and photovol- taic performance of the solar cells made of those molecules. Although several new high-performance donor materials have emerged in the past few years, the development of nonfullerene acceptors (NFAs) is still in the infancy. [4] The most successful and commonly used elec- tron acceptors till date are fullerene and its soluble derivative [6-6]-phenyl-C 61 / C 71 -butyric acid methyl ester (PC 61 BM/ PC 71 BM). [5] However, the main drawbacks associated with the fullerene based accep- tors are their weak light absorption in the visible range of the solar spectrum, wide bandgap, and high synthesis cost. [6,7] In contrast, the NFAs can have several advantages, such as tunable molecular structure and energy levels, wide absorp- tion range extended toward visible and near IR region of solar spectrum, facile synthesis, easy purifications, and low cost. The most widely investigated struc- tural templates for NFAs are perylenedi- imide derivatives, [4,8–10] benzothiadiazole, [11] dicyanovinyl, [12–16] diketopyrrolopyrrole, [7,16,17] and rhodanines. [18–20] Another promising NFA design is fluorine core flanked with the wings of 3-ethylrhodanine, indandione, thiophene-2-carbonitrile or, malononitrile. A high of PCE 7.16% was achieved from a perylene bisimide (PBI) dimer based acceptor SdiPBI-S, devel- oped by Sun et al., [21] when blended with a wide bandgap donor polymer PDBT-T1. Several electron acceptor moie- ties, such as corannulene and truxenone having rotationally symmetric polycyclic aromatic cores comprising 5th and 6th membered ring have been developed for solution processable organic photovoltaic (OPV) devices. [22] A phthalocyanine based rotationally symmetric acceptor, labeled as Cl 6 -SubPc-Cl, exhibited PCE of 6.86% with SubNc donor. [23] Calamitic shaped small molecules have received immense attention recently as an effective design strategy to develop a class of high-performance NFAs. The acceptor 3,9-bis(2-methylene- (3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis (4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′] dithiophene (ITIC) and donor poly[(2,6-(4,8-bis(5-(2-ethylhexyl) thiophene-2-yl)-benzo[1,2-b:4,5-b ′ ]dithiophene))-alt- (5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′] dithiophene-4,8-dione))] (PBDB-T) based OPV device achieved Molecular Engineering of Highly Efficient Small Molecule Nonfullerene Acceptor for Organic Solar Cells Suman, Vinay Gupta,* Anirban Bagui, and Surya Prakash Singh* A new molecularly engineered nonfullerene acceptor, 2,2′-(5,5′-(9,9-didecyl-9H-fluorene-2,7-diyl)bis(benzo[c][1,2,5]thiadiazole- 7,4-diyl)bis(methanylylidene))bis(3-hexyl-1,4-oxothiazolidine-5,2-diylidene)) dimalononitrile (BAF-4CN), with fluorene as the core and arms of dicyano- n-hexylrhodanine terminated benzothiadiazole is synthesized and used as an electron acceptor in bulk heterojunction organic solar cells. BAF-4CN shows a stronger and broader absorption with a high molar extinction coefficient of 7.8 × 10 4 M -1 cm -1 at the peak position (498 nm). In the thin film, the molecule shows a redshift around 17 nm. The photoluminescence experiments confirm the excellent electron accepting nature of BAF-4CN with a Stern–Volmer coefficient (K sv ) of 1.1 × 10 5 M -1 . From the electro- chemical studies, the highest occupied molecular orbital and lowest unoc- cupied molecular orbital energy levels of BAF-4CN are estimated to be -5.71 and -3.55 eV, respectively, which is in good synchronization with low bandgap polymer donors. Using BAF-4CN as an electron acceptor in a poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3″′-di(2-octyldodecyl) 2,2′;5′,2″;5″,2″′-quaterthiophen-5,5″′-diyl)] based bulk-heterojunction solar cell, a maximum power conversion efficiency of 8.4% with short-circuit cur- rent values of 15.52 mA cm -2 , a fill factor of 70.7%, and external quantum efficiency of about 84% covering a broad range of wavelength is achieved. DOI: 10.1002/adfm.201603820 Suman, Dr. A. Bagui, Dr. S. P. Singh Inorganic and Physical Chemistry Division CSIR-Indian Institute of Chemical Technology (IICT) Uppal Road, Tarnaka, Hyderabad 500007, India E-mail: spsingh@iict.res.in Suman, Dr. S. P. Singh Academy of Scientific and Innovative Research (AcSIR) New Delhi 110 025, India Dr. V. Gupta Organic and Hybrid Solar Cell Group CSIR-National Physical Laboratory Dr. K. S. Krishnan Marg, New Delhi 110012, India E-mail: drvinaygupta@netscape.net 1. Introduction Molecular engineering of photoactive organic materials for organic solar cell applications has emerged as a rapidly growing research topic in recent years. Power conversion efficiencies (PCEs) over 10% have been achieved from the organic solar cells (OSCs) built with newly designed small- molecule and polymeric materials. [1–3] The structural, optical, www.afm-journal.de Adv. Funct. Mater. 2017, 27, 1603820 www.advancedsciencenews.com